1. Field of the Invention
The present invention relates to apparatuses required to monitor imbalance between two branches of signals in quadrature to each other, and more particularly to a phase imbalance monitoring apparatus and an amplitude imbalance monitoring apparatus in an optical communication system, and a receiver incorporating a phase imbalance monitoring apparatus or an amplitude imbalance monitoring apparatus.
2. Description of the Related Art
With the gradual enhancement on the requirements of capacity and flexibility of the optical communication system, the coherent optical communication technology has become more and more important. In comparison with incoherent technology (such as on-off key, OOK) or self-coherent technology (such as differential quadrature phase-shift keying, DQPSK) already widely applied in the field of optical communications, the coherent technology has the following advantages: optical signal-to-noise ratio (OSNR) gain of 3 dB; the capability to use equalization technologies; and the capability to use more efficient modulation technologies (such as quadrature modulation, QAM).
One important device in the coherent receiver is the hybird, such as the product provided by Optoplex Corporation. Due to limitations of the devices, the phase between the in-phase branch output I and the quadrature branch output Q of the hybird is not exactly 90°, and the offset is referred to as IQ phase imbalance. In addition, the powers of the two branches are also slightly different, and the offset is referred to as IQ amplitude imbalance. Such imbalances would degrade the system performance, so that it is necessary for the coherent receiver to detect the imbalances and compensate for them.
FIG. 1 is a schematic structural view that exemplarily illustrates a coherent receiver according to the state of the art. As shown in FIG. 1, the receiver comprises an optical 90° hybird 102, a local oscillating section 103, PD sections 104 and 105, a recover 106, an I/Q phase imbalance monitoring section 107, an I/Q amplitude imbalance monitoring section 109, and control sections 108 and 110.
A received optical signal 101 and an output of the local oscillating section 103 are inputted to the optical 90° hybird 102. The optical 90° hybird 102 generates four optical signals, namely S+L, S−L, S+jL and S−jL, wherein S is the received optical signal, L is the output of the local oscillating section 103, and j indicates 90° phase shift. For instance, the optical 90° hybird 102 can be a product provided by Celight Corporation. The PDs 104 and 105 convert the four optical signals into two electric signals. Specifically, for example, PD 104 converts S+L and S-L into an I branch signal, and PD 105 converts S+jL and S−jL into a Q branch signal. The I branch signal and the Q branch signal should be in quadrature to each other in principle. The recover 106 recovers data in the I branch and Q branch signals by means of a carrier phase recover, a match filter, and a data recover, etc. The recover can be implemented either in the analog domain or in the digital domain. As noted above, due to reasons such as the hardware performance of the optical 90° hybird 102, phase imbalance and amplitude imbalance might occur in the I branch and Q branch signals. As shown in FIG. 1, the I/Q phase imbalance monitoring section 107 monitors the phase imbalance, while the I/Q amplitude imbalance monitoring section 109 monitors the amplitude imbalance. Moreover, the control sections 108 and 110 respectively control the optical 90° hybird 102 in accordance with the monitoring results of the I/Q phase imbalance monitoring section 107 and the I/Q amplitude imbalance monitoring section 109, so as to compensate the detected phase imbalance and amplitude imbalance. Specifically, when the I/Q phase imbalance is positive, the control section outputs a positive voltage to a phase control port of the optical 90° hybird to reduce phase shift of the Q branch (or I branch), and vice versa.
In the detection of phase imbalance and amplitude imbalance, the technology proposed in “Digital filter equalization of analog gain and phase mismatch in I-Q receivers” Fred Harris, 5th IEEE International Conference on Universal Personal Communications, 1996, and the technology proposed in U.S. Pat. No. 6,917,031 B1 “method for quadrature phase angle correction in a coherent receiver of a dual-polarization optical transport system” have been in current used. Both of the two calculate correlation between I and Q in the digital domain, wherein when phase imbalance is zero, correlation is also zero, and the correlation is in direct proportion to the phase imbalance. Harris' paper makes use of feedback digital compensation loop, while the U.S. Pat. No. 6,917,031 B1 makes use of feed forward digital compensation. Harris' paper also provides an IQ amplitude imbalance detection and compensation method. All the foregoing methods require complicated digital signal processing, such as multiplication and squaring of complex numbers. The bit rate of certain systems (such as the optical communication system) is as high as 43 Gbit/s. It is very difficult to perform digital signal processing under such high speed signals, so that the methods of the conventional electric communication systems as mentioned above are difficult for application.
In addition, in terms of analog signals, since the capacity of the optical communication system can be as high as 43 Gbit/s, the bandwidths of the I branch and the Q branch can reach as high as 20 GHz or more. Accordingly, if the methods of the aforementioned documents were directly applied to the analog signals, a bandwidth of approximately 20 GHz would be required for the multiplier of the relevant devices, and such an analog multiplier is very difficult for implementation.